GRIN plasmonics

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They said it could be done and now they've done it. What's more,
they did it with a GRIN. A team of researchers with the U.S.
Department of Energy (DOE)'s Lawrence Berkeley National Laboratory
(Berkeley Lab) and the University of California, Berkeley, have
carried out the first experimental demonstration of GRIN —
for gradient index — plasmonics, a hybrid technology that
opens the door to a wide range of exotic optics, including
superfast computers based on light rather than electronic signals,
ultra-powerful optical microscopes able to resolve DNA molecules
with visible light, and "invisibility" carpet-cloaking devices.

Working with composites featuring a dielectric (non-conducting)
material on a metal substrate, and "grey-scale" electron beam
lithography, a standard method in the computer chip industry for
patterning 3-D surface topographies, the researchers have
fabricated highly efficient plasmonic versions of Luneburg and
Eaton lenses. A Luneburg lens focuses light from all directions
equally well, and an Eaton lens bends light 90 degrees from all
incoming directions.

"This past year, we used computer simulations to demonstrate
that with only moderate modifications of an isotropic dielectric
material in a dielectric-metal composite, it would be possible to
achieve practical transformation optics results," says Xiang Zhang,
who led this research. "Our GRIN plasmonics technique provides a
practical way for routing light at very small scales and producing
efficient functional plasmonic devices."

Zhang, a principal investigator with Berkeley Lab's Materials
Sciences Division and director of UC Berkeley's Nano-scale Science
and Engineering Center (SINAM), is the corresponding author of a
paper in the journal Nature Nanotechnology, describing this
work titled, "Plasmonic Luneburg and Eaton Lenses." Co-authoring
the paper were Thomas Zentgraf, Yongmin Liu, Maiken Mikkelsen and
Jason Valentine.

GRIN plasmonics combines methodologies from transformation
optics and plasmonics, two rising new fields of science that could
revolutionize what we are able to do with light. In transformation
optics, the physical space through which light travels is warped to
control the light's trajectory, similar to the way in which outer
space is warped by a massive object under Einstein's relativity
theory. In plasmonics, light is confined in dimensions smaller than
the wavelength of photons in free space, making it possible to
match the different length-scales associated with photonics and
electronics in a single nanoscale device.

"Applying transformation optics to plasmonics allows for precise
control of strongly confined light waves in the context of
two-dimensional optics," Zhang says. "Our technique is analogous to
the well-known GRIN optics technique, whereas previous plasmonic
techniques were realized by discrete structuring of the metal
surface in a metal-dielectric composite."

Like all plasmonic technologies, GRIN plasmonics starts with an
electronic surface wave that rolls through the conduction electrons
on a metal. Just as the energy in a wave of light is carried in a
quantized particle-like unit called a photon, so, too, is plasmonic
energy carried in a quasi-particle called a plasmon. Plasmons will
interact with photons at the interface of a metal and dielectric to
form yet another quasi-particle, a surface plasmon polariton
(SPP).

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The Luneburg and Eaton lenses fabricated by Zhang and his
co-authors interacted with SPPs rather than photons. To make these
lenses, the researchers worked with a thin dielectric film (a
thermplastic called PMMA) on top of a gold surface. When applying
grey-scale electron beam lithography, the researchers exposed the
dielectric film to an electron beam that was varied in dosage
(charge per unit area) as it moved across the film's surface. This
resulted in highly controlled differences in film thickness across
the length of the dielectric that altered the local propagation of
SPPs. In turn, the "mode index," which determines how fast the SPPs
will propagate, is altered so that the direction of the SPPs can be
influenced.

"By adiabatically tailoring the topology of the dielectric layer
adjacent to the metal surface, we're able to continuously modify
the mode index of SPPs," says Zentgraf. "As a result, we can
manipulate the flow of SPPs with a greater degree of freedom in the
context of two-dimensional optics."

Says Liu, "The practicality of working only with the purely
dielectric material to transform SPPs is a big selling point for
GRIN plasmonics. Controlling the physical properties of metals on
the nanometer length-scale, which is the penetration depth of
electromagnetic waves associated with SPPs extending below the
metal surfaces, is beyond the reach of existing nanofabrication
techniques."

Adds Zentgraf, "Our approach has the potential to achieve
low-loss functional plasmonic elements with a standard fabrication
technology that is fully compatible with active plasmonics."

In the Nature Nanotechnology paper, the researchers say
that inefficiencies in plasmonic devices due to SPPs lost through
scattering could be reduced even further by incorporating various
SPP gain materials, such as fluorescent dye molecules, directly
into the dielectric. This, they say, would lead to an increased
propagation distance that is highly desired for optical and
plasmonic devices. It should also enable the realization of
two-dimensional plasmonic elements beyond the Luneburg and Eaton
lenses.

Says Mikkelsen, "GRIN plasmonics can be immediately applied to
the design and production of various plasmonic elements, such as
waveguides and beam splitters, to improve the performance of
integrated plasmonics. Currently we are working on more complex,
transformational plasmonic devices, such as plasmonic collimators,
single plasmonic elements with multiple functions, and plasmonic
lenses with enhanced performance."

(DOE/Lawrence Berkeley National Laboratory) Berkeley Lab researchers have carried out the first experimental demonstration of GRIN plasmonics,a hybrid technology that opens the door to a wide range of exotic applications in optics, including superfast photonic computers, ultra-powerful...